1. Field of the Invention
The present invention relates to an optical recording medium, and in particular, to an optical recording medium having a land-and-groove structure, wherein pits, by which address information of the grooves is recorded, are formed in advance in the lands.
2. Description of the Related Art
In recent years, along with the development of short wavelength lasers, digital video discs (DVDs) have come to be used. Information can be recorded on DVDs at a higher density than on compact discs (CDs), and the recorded information can be played back. Currently, DVD-Rs, which are writable optical recording media on which a user can write information, are also in use.
A writable optical recording medium is usually provided with a substrate, a recording layer, a reflecting layer, and a protective layer. Grooves for tracking during recording are provided in the substrate in advance. The regions adjacent the grooves are called lands. Pits called land pre-pits (LPPs) are formed in advance in the lands of a DVD-R. The address information of the groove at the inner peripheral side of the land is thereby recorded at the land.
When the information recorded on a DVD-R is played back by tracking the grooves having pits therein by a push-pull method, the return light from the beam spot collected on the groove is converted into electric signals by four diodes A, B, C, D, so as to obtain signals A, B, C, D corresponding to the respective diodes. The value (A+B+C+D) which is the sum of the signals A, B, C, D is the playback signal for the recorded information. The value (A+Bxe2x88x92Cxe2x88x92D), which is the signals A, B corresponding to the return light at the outer peripheral side minus the signals C, D corresponding to the return light at the inner peripheral side, is the tracking error signal. Accordingly, as illustrated in FIG. 7, a negative pulse, which corresponds to the LPP of the land at the outer peripheral side of the groove, and a positive pulse, which corresponds to the LPP of the land at the inner peripheral side of the groove, are present in the tracking error signal. Because the address information of the groove at the inner peripheral side of a land is recorded in that land, by detecting the LPP signal expressed as the negative pulse, the address information recorded by the LPP can be read out.
In order to accurately read the address information, the negative pulse must be strong, and the surface area of the portion of the LPP at the inner peripheral side thereof must be made large to a certain extent. Further, the LPP is usually substantially oval, and is formed such that the center of the oval is positioned substantially on the center line of the land disposed between the grooves. Thus, as shown in FIG. 8, the LPP is usually open to (continuous with) both of the grooves adjacent to the LPP.
However, because the positive pulse is not used in reading the address information, there is no need for the positive pulse to be strong. Conversely, the positive pulse being too strong results in noise with respect to the Rf signal. Further, when the LPP opens onto the grooves, at the time the pit is formed in the recording layer on the region adjacent to the LPP, the material of the recording layer swells and spreads as far as the LPP such that the desired pit length cannot be obtained, and information cannot be recorded correctly.
Accordingly, an object of the present invention is to provide an optical recording medium in which address information can be read correctly, while good recording/playback characteristics of the optical recording medium are still maintained.
In order to achieve this object, the present invention provides an optical recording medium having a substrate which has a spiral or concentric circular grooves and has lands between the grooves, wherein a pit is formed in advance in the land, the pit being open only toward one groove of the grooves adjacent to the land in which the pit is formed, address information of the one groove toward which the pit is open being recorded by the pit.
In the present invention, the pits (LPPs) formed in the land are open toward only one groove of the grooves adjacent the land. The pits are not open toward the other groove. Thus, when pits are formed in the recording layer on the other groove, the land in which the LPPs are formed can prevent the recording layer material from spreading to the LPPs. As a result, the LPP can be prevented from adversely affecting the length of the pit formed in the recording layer of the other groove.
If the surface area of the LPP at the open side thereof is made sufficiently large, the pulse of the LPP signal, which pulse is necessary for reading the address information, is sufficiently strong. Further, when a beam spot is illuminated onto the groove at the side at which the LPP is not open, the pulse disused for reading the address information does not exist (or is small). As a result, the address information can be read accurately, and noise with respect to the RF signal can be reduced.